Selective release of muscle-specific, extracellular microRNAs during myogenic differentiation

Abstract

MyomiRs are muscle-specific microRNAs (miRNAs) that regulatemyoblast proliferation and differentiation. Extracellular myomiRs (ex-myomiRs) are highly enriched in the serum of Duchenne Muscular Dystrophy (DMD) patients and dystrophic mousemodels and consequently have potential as disease biomarkers. The biological significance ofmiRNAs present in the extracellular space is not currently well understood. Here we demonstrate that ex-myomiR levels are elevated in perinatal muscle development, during the regenerative phase that follows exercise-inducedmyoinjury, and concomitant with myoblast differentiation in culture.Whereas ex-myomiRs are progressively and specifically released by differentiating human primary myoblasts and C2C12 cultures, chemical induction of apoptosis in C2C12 cells results in indiscriminatemiRNA release. The selective release ofmyomiRs as a consequence of cellular differentiation argues against the idea that they are solely waste products ofmuscle breakdown, and suggests they may serve a biological function in specific physiological contexts. Ex-myomiRs in culture supernatant and serum are predominantly non-vesicular, and their release is independent of ceramide-mediated vesicle secretion. Furthermore, ex-myomiRs levels are reduced in aged dystrophicmice, likely as a consequence of chronicmuscle wasting. In conclusion, we show that myomiR release accompanies periods ofmyogenic differentiation in cell culture and in vivo. SerummyomiR abundance is therefore a function of the regenerative/degenerative status of themuscle, overallmuscle mass, and tissue expression levels. These findings have implications for the use of ex-myomiRs as biomarkers for DMD disease progression and monitoring response to therapy.